AMERICAN NATIONAL STANDARD

Transcription

1 Interface Practices Subcommittee AMERICAN NATIONAL STANDARD Test Procedure for Hum Modulation

2 NOTICE The Society of Cable Telecommunications Engineers (SCTE) Standards and Operational Practices (hereafter called documents ) are intended to serve the public interest by providing specifications, test methods and procedures that promote uniformity of product, interchangeability, best practices and ultimately the long term reliability of broadband communications facilities. These documents shall not in any way preclude any member or non-member of SCTE from manufacturing or selling products not conforming to such documents, nor shall the existence of such standards preclude their voluntary use by those other than SCTE members. SCTE assumes no obligations or liability whatsoever to any party who may adopt the documents. Such adopting party assumes all risks associated with adoption of these documents, and accepts full responsibility for any damage and/or claims arising from the adoption of such documents. Attention is called to the possibility that implementation of this document may require the use of subject matter covered by patent rights. By publication of this document, no position is taken with respect to the existence or validity of any patent rights in connection therewith. SCTE shall not be responsible for identifying patents for which a license may be required or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention. Patent holders who believe that they hold patents which are essential to the implementation of this document have been requested to provide information about those patents and any related licensing terms and conditions. Any such declarations made before or after publication of this document are available on the SCTE web site at All Rights Reserved Society of Cable Telecommunications Engineers, Inc Philips Road Exton, PA AMERICAN NATIONAL STANDARD SCTE 2

3 Title Table of Contents Page Number NOTICE 2 Table of Contents 3 1. Introduction 4 2. Normative References 5 3. Informative References 5 4. Compliance Notation 6 5. Abbreviations and Definitions 6 6. Background 7 7. Equipment 7 8. Setup db Delta Measurement Procedure Differential Voltage Measurement Procedure 12 Appendix 1 Hum Modulation Derivation 14 Appendix 2 Derivation of the 100mVDC Delta Calibration 16 Appendix 3 Conversion of Hum Modulation from db to percent (%) 18 Appendix 4 Time Domain vs. Frequency Domain Measurements 20 Appendix 5 - Hum Modulation Test Methods 21 Appendix 6 - Troubleshooting Guide 23 Appendix 7 Special Test Conditions/Considerations 24 List of Figures Title Page Number Figure 1 - Hum Modulation Test Setup 13 Figure 2 Continuous Wave Hum Modulation 15 List of Tables Title Page Number Table 1 - Hum Modulation Conversion Chart 19 AMERICAN NATIONAL STANDARD SCTE 3

4 1. Introduction 1.1. Executive Summary This test procedure provides instructions for how to measure hum modulation in active and passive broadband RF telecommunications equipment (taps, line passives, RF Amplifiers, Optical Nodes, etc.) in a laboratory environment. This procedure is not intended for field measurements Scope The purpose of this test procedure is to define and measure hum modulation in active and passive broadband RF telecommunications equipment and sub-assemblies. This procedure presents two methods for measuring hum modulation in the time domain, with a sensitivity exceeding -80 db. These methods are referred to as the 1 db delta and the differential voltage method. A mathematical relationship between time domain and frequency domain measurement methods is also provided. This document provides specifications or procedures for frequencies up to 1002 MHz. DOCSIS 3.1 specifications include operation at frequencies up to 1218 MHz, and optionally, to 1794 MHz. Specifications or procedures for those higher frequencies should be considered in a future revision of this document Benefits In a coaxial telecommunications system, AC power and radio frequency (RF) signals can exist on the same conductor. As a result, amplitude modulation of the transmitted signals at the power line frequency (hum modulation) can occur in both active and passive equipment due to excessive ac voltage at the output of a local power pack (in the case of an active device) or from parametric modulation of magnetic components (used in circuitry to separate power and signal voltages) in active or passive devices. In North American NTSC analog television systems, the frame repetition rate is Hz and the commercial power frequency is 60 Hz. Amplitude modulation of the transmitted signals at the power line frequency (hum modulation) can cause picture disruption in the form of horizontal bars or brightness variation. Excessive hum modulation could also disrupt AGC circuitry commonly used in RF amplifiers, causing excessive RF level variations in the HFC plant. The FCC (Title 47, Part ) requires that hum modulation does not exceed 3% of the visual signal level. This test procedure for hum modulation, when executed according the procedure, will yield confidence in the measured hum modulation performance of the active or passive broadband RF telecommunications equipment under test. Use of this test procedure provides a means for the user to verify acceptable performance of the unit under test Intended Audience The intended audience for this test procedure includes manufacturers, corporate engineers, technical operations, and technicians Areas for Further Investigation or to be Added in Future Versions None at the time of update. AMERICAN NATIONAL STANDARD SCTE 4

5 Specifications or procedures for frequencies above 1002 MHz should be considered in a future revision of this document. 2. Normative References The following documents contain provisions, which, through reference in this text, constitute provisions of this document. At the time of Subcommittee approval, the editions indicated were valid. All documents are subject to revision; and while parties to any agreement based on this document are encouraged to investigate the possibility of applying the most recent editions of the documents listed below, they are reminded that newer editions of those documents might not be compatible with the referenced version SCTE References ANSI/SCTE 96, 2013, Cable Telecommunications Testing Guidelines 2.2. Standards from Other Organizations No normative references are applicable Published Materials No normative references are applicable. 3. Informative References The following documents might provide valuable information to the reader but are not required when complying with this document SCTE References ANSI/SCTE 82, 2012, Test Method for Low Frequency and Spurious Disturbances 3.2. Standards from Other Organizations 47eCFR76, Electronic Code of Federal Regulations (e-cfr), Title 47, Telecommunication, Part 76, Subpart K, Multichannel Video and Cable Television Service 3.3. Published Materials Ciciora, Farmer, Large, Adams. (2004) Modern Cable Television Technology (2 nd Edition). San Francisco: Morgan Kaufmann. P , 631 AMERICAN NATIONAL STANDARD SCTE 5

6 4. Compliance Notation shall shall not forbidden should should not may deprecated 5. Abbreviations and Definitions 5.1. Abbreviations This word or the adjective required means that the item is an absolute requirement of this document. This phrase means that the item is an absolute prohibition of this document. This word means the value specified shall never be used. This word or the adjective recommended means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighted before choosing a different course. This phrase means that there may exist valid reasons in particular circumstances when the listed behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label. This word or the adjective optional means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item. Use is permissible for legacy purposes only. Deprecated features may be removed from future versions of this document. Implementations should avoid use of deprecated features. AC AM db DC DUT FCC GHz HFC Hz khz ms mv RF SCTE VSWR 5.2. Definitions Hum Modulation alternating current amplitude modulation decibel direct current device under test Federal Communications Commission gigahertz hybrid fiber-coax hertz kilohertz millisecond millivolt radio frequency Society of Cable Telecommunications Engineers voltage standing wave ratio The amplitude distortion of a signal caused by the modulation of the signal with components of the power source. It is the ratio, expressed in db, of the peak to peak variation of the carrier level caused by AC power line frequency products (and harmonics up to 1 khz) to the peak voltage amplitude of the carrier. AMERICAN NATIONAL STANDARD SCTE 6

7 Peak Voltage Amplitude Percent Hum Modulation The maximum voltage amplitude of the carrier. Hum Modulation may also be expressed as a percentage of the peak to peak variation of the carrier level to the peak voltage amplitude of the carrier. See Appendix 3 for a derivation of percent Hum. 6. Background In a coaxial telecommunications system, where AC power and radio frequency (RF) signals exist on the same conductor, hum modulation of a television signal is measured as a comparison between the modulation envelope (created by line power distortions of the RF carriers), and the peak voltage of the sync tips of the video signal. In a laboratory test environment, the video signal is replaced with a sine wave RF carrier. Under these conditions, the only modulation existing on the carrier is the line power related distortion (created by the device under test) of that carrier. Hum modulation is determined as a function of the line frequency current passing through the device by comparing the peak to peak modulation envelope of the power line distortion to the peak voltage of the carrier by utilizing a diode detector. Due to the non-linear characteristics of any diode detector, it is inaccurate to simply compare the AC modulation voltage and the rectified DC carrier voltage directly due to the voltage drop across the diode. Instead, the modulation voltage is compared to a calibrated change in the rectified carrier. There are two time-domain methods for measuring hum modulation in a laboratory environment, which are detailed in this procedure. These methods are commonly referred to as the 1 db delta method, and the differential voltage method. These methods are described in Appendix Equipment 7.1. The general equipment required for this test is shown in Figure 1. ANSI/SCTE 96 (Cable Telecommunications Testing Guidelines), describes and specifies this equipment The signal generator for this test must have (minimum) the characteristics listed below MHz GHz (minimum) single carrier signal generation capability % minimum AM modulation index capability < 0.01% residual hum Two AC/RF Power Combiners/Inserters: Hum modulation < -80 db at desired test current and voltage Return loss: > 22 db Power Combiners/Inserters must possess RF only input/output capability, i.e. AC blocking incorporated into design Current carrying capacity of at least 50% greater than desired test currents. Note: AC/RF Power Combiners/Inserters may not be required for certain DUT applications. Refer to Appendix 7 for special test considerations. AMERICAN NATIONAL STANDARD SCTE 7

8 7.4. Resistive Load Bank: capable of dissipating desired test power Display Devices: Power analyzer capable of measuring rms current and voltage simultaneously Oscilloscope must have a minimum deflection sensitivity of 2.0 mv/div The oscilloscope used for the test must possess signal averaging capability Power supply: Ferro-resonant quasi-square wave (trapezoidal wave with < 100 V/ms slew rate) AC voltage output (50/60 Hz) at desired test voltage Rated current output of at least 30% greater than the desired test current Low Pass Filter: DC 1 khz(minimum) Shielded High Impedance (5 kω or greater) > 30 db attenuation above 30 khz This filter may be contained in the differential preamplifier used in the test AM Detector: Ω Input Impedance Frequency Range: 200 khz to GHz Maximum VSWR: 1.2:1 to GHz Required inter-connect cables and connectors Quad-shield 75Ω coaxial cable, for RF only paths Rigid or semi-rigid 75Ω coaxial cable (1/2 diameter or greater), for RF and AC common paths, with a current carrying capacity of at least 50% greater than desired test currents Isolation Transformer VAC Input : VAC Output /60 Hz > 1,000 VA RF Amplifier AMERICAN NATIONAL STANDARD SCTE 8

9 MHz 1 GHz (two amplifiers may be used to encompass this frequency range) Gain 25 db Noise Figure 10 db RF Attenuators 8. Setup Maximum VSWR: 1.2:1 to 1 GHz Ω Switchable Attenuators db steps, 0 1 db range, ±0.05 db tolerance db steps, 0 10 db range, ±0.1 db tolerance db steps, 0 70 db range, ±0.2 db tolerance Ω fixed attenuators: 10 db values, ±0.05 db tolerance Follow all calibration requirements recommended by the manufacturers of the test equipment, including adequate warm-up and stabilization time Connect the test equipment as shown in Figure 1. Note: AC/RF Power Combiners/Inserters may not be required for certain DUT applications. Refer to Appendix 7 for special test considerations Set the oscilloscope to the following settings: Vertical Scale As required for waveform display Horizontal Scale Input Impedance AC Coupled Mode Measurement Mode Signal Averaging Trigger 1 ms/div 1 MΩ On V pk-pk Off External Note: A pre-amplifier is recommended to increase the vertical resolution of the oscilloscope, and extend the noise floor. AMERICAN NATIONAL STANDARD SCTE 9

10 8.4. Determine RF levels: Connect DUT into test setup Set the signal generator to a frequency within the operating range of the device Adjust the signal generator RF output level, while monitoring the RF test point, to set the RF output level of the DUT to it s typical operating levels Adjust the step attenuator preceding the RF post amplifier until the voltmeter displays approximately 0.9 VDC. If this voltage cannot be obtained, the gain of the RF post amp will need to be increased. This will ensure operation in the linear region of the AM detector. Note: Section 8.4 establishes the calibration voltage while maintaining the DUT at its rated output level (achieved by monitoring the test point). This prevents possible compression of the DUT while obtaining the calibration voltage Calibration: Add 1.0 db of attenuation from the input attenuator and note the voltage change on the voltmeter Fine tune the amplitude of the signal generator until a 1.0 db switchable attenuator change causes a 100 mvdc deflection on the voltmeter. When a 100 mvdc delta is achieved, record the DC voltage at the higher signal level as the detector dc operating point or calibration voltage.(see Appendix 2) 8.6. Calibration verification: AM modulate the signal generator at 0.5%(conventional AM index), 400 Hz Adjust oscilloscope for maximum signal deflection, then measure Vp-p and signal period. With the pre-amplifier gain set to 100, a sinusoidal signal with a period of 2.5 ms (400 Hz) and amplitude of approximately 900 mv should appear on the oscilloscope if the setup is properly calibrated. This corresponds to a hum modulation of 1.0% or -40 db (see section 8.7 and Appendix 2 for calculations) Turn off signal generator AM modulation AC Power Combiner Back-to-Back Hum Modulation Characterization: Remove the DUT and install a calibration jumper as shown in Figure 1. A cable or adapter may be used in place of the DUT, when applicable Turn on AC power supply and adjust load bank to desired test current, as displayed on power analyzer Set signal generator to desired test frequency Set-up the oscilloscope: Horizontal Scale 5 ms/div AMERICAN NATIONAL STANDARD SCTE 10

11 Trigger Signal Averaging Line Off Adjust RF amplitude at the generator until the voltmeter displays the calibration voltage determined in section 8.5.2, above. Record this voltage in millivolts as Va Adjust the oscilloscope for maximum deflection of the non-averaged signal, making sure that none of the signal amplitude goes off-screen Set signal averaging to 64 samples or greater Record the peak-to-peak voltage of the hum modulation waveform as mvp-p in millivolts after the averaging has settled Add 1 db of attenuation at the generator output or decrease the signal generator level by 1 db and record the DC voltage in millivolts displayed by the voltmeter as Vb Calculate noise floor hum in db as: Hum (db) = log(Va-Vb) + 20log(mVp-p/100) (See Appendix 2 for derivation) Note: The mvp-p value must be divided by 100 due to the pre-amplifier gain. If the pre-amplifier is not used, the mvp-p value is divided by Calculate hum in percent as: Hum (%) = 100 * 10Hum(dB)/20 (See Appendix 3 for derivation) Repeat steps through for all test frequencies Turn off power supply and remove calibration jumper Turn off signal generator RF output. Note: The back-to-back hum modulation of the power combiners should be a minimum of 75 db, with a goal of greater than 80 db. If the noise floor is not lower than 75 db, there may be a ground loop in the AC circuit. See Appendix 6 for ground loop trouble shooting recommendations db Delta Measurement Procedure 9.1. Connect DUT into test setup Turn on power supply and load to desired test current (using the resistive load bank) as measured on the power analyzer Operate the DUT under power until it reaches temperature stability Set signal generator to desired test frequency and turn RF on Adjust RF amplitude at the generator until the voltmeter displays the calibration voltage determined in section 8.5.2, above. Record this voltage as Va. (in mv) AMERICAN NATIONAL STANDARD SCTE 11

12 9.6. Adjust the oscilloscope for maximum deflection of the non-averaged signal, making sure that none of the signal amplitude goes off-screen Set signal averaging to 64 samples or greater Record the peak-to-peak voltage of the hum modulation waveform as mvp-p in millivolts after the averaging has settled. Note: When using signal averaging, make sure that the non-averaged signal does not go off the screen when adjusting the oscilloscope for maximum deflection. This can cause severe averaging errors Decrease the signal generator level by 1.0 db and record the DC voltage displayed by the voltmeter as Vb. (in mv) Calculate hum in db as: Hum (db) = log(Va-Vb) + 20log(mVp-p/100). (See Appendix 2 for derivation) Note: The mvp-p value must be divided by 100 due to the pre-amplifier gain. If the pre-amp is not used, the mvp-p value is divided by Calculate hum in percent as: Hum (%) =100 * 10Hum(dB)/20.(See Appendix 3 for derivation) Repeat steps 9.4 through 9.11 for all test frequencies and temperatures. 10. Differential Voltage Measurement Procedure Connect DUT into test setup Turn on power supply and load to desired test current (using the resistive load bank) as measured on the power analyzer Operate the DUT under power until it reaches temperature stability Set signal generator to desired test frequency and turn RF on Adjust RF amplitude at the generator until the voltmeter displays the calibration voltage determined in section 8.5.2, above. Note: The signal generator level can be adjusted to levels less than the calibration voltage without loss of accuracy to accommodate measurements such as a sloped frequency response at the DUT output. The generator level does not need to be adjusted at each frequency if the calibration voltage is not exceeded during the test sequence AM modulate the signal generator to 0.5 % (conventional AM index) at 400 Hz Set the oscilloscope to the following settings: Vertical Scale Horizontal Scale Measurement Mode AC Coupled Mode Signal Averaging As required for waveform display 1 ms/div V pk-pk On > 64 Samples AMERICAN NATIONAL STANDARD SCTE 12

13 Trigger External Note: Reliable triggering of the oscilloscope is obtained through a direct connection between the signal generator modulation test point and the oscilloscope external trigger. Do not make a ground connection between the signal generator and oscilloscope through this cable as this will cause ground loop hum The oscilloscope should display the reference modulation envelope. Record the peak to peak amplitude value as Vref in mvp-p when the averaging has settled Turn off signal generator AM modulation Change the following oscilloscope settings: Horizontal Scale Trigger 5 ms/div Line Adjust the oscilloscope for maximum deflection of the displayed signal and record the peak to peak amplitude value as Vhum in mvp-p when the averaging has settled. Note: When using signal averaging, make sure that the non-averaged signal does not go off the screen when adjusting the oscilloscope for maximum deflection. This can cause severe averaging errors Calculate hum in db as: Hum (db) = log(Vref) + 20log(Vhum). (See Appendix 5 for derivation.) Calculate hum in % as: Hum (%) = 100 * 10Hum(dB)/20. (See Appendix 3 for derivation.) Repeat steps 10.4 through for all test frequencies and temperatures. Figure 1 - Hum Modulation Test Setup AMERICAN NATIONAL STANDARD SCTE 13

14 Appendix 1 Hum Modulation Derivation The equation for a TV sinewave amplitude modulated carrier is: y(t) = B[1 (k / 2)(1 + cosω m t)]cosω c t (A -1) where B is the peak amplitude of the RF carrier and k is the modulation factor which is equal to: k = M pk B pk, k 1 (A - 2) and k is related to the conventional AM modulation index by the equation: 2m k = where m is the conventional AM modulation index. 1+ m ( ) If the peak-to-peak amplitude variation in time is kb, and this amplitude variation is considered to be hum modulation, hum is defined as. AM Mod(dB) = 20Log kb = 20Log k (A - B ( ) ( ) 3) Hum modulation is simply a measurement of k, which is the ratio of the peak-to-peak hum on the carrier to the peak of the RF carrier (B). k = Hum B pk pk (A - 4) Figure 2 shows a graphical representation of hum modulation using a continuous wave (CW) carrier as an example. AMERICAN NATIONAL STANDARD SCTE 14

15 Figure 2 Continuous Wave Hum Modulation AMERICAN NATIONAL STANDARD SCTE 15

16 Appendix 2 Derivation of the 100mVDC Delta Calibration The 100mVDC delta hum modulation reference calibration procedure establishes, through a 1 db RF level change, a repeatable envelope change that can be compared to the actual peak-to-peak hum voltage. From this a hum modulation ratio can be determined. Consider the signal: v(t) = B 1 v (t) cosω [ ] t (A - 5) hum c Where B is the peak amplitude of the RF carrier, v hum(t) is the peak-to-peak hum voltage, and B in db is defined as B(dB) = 20log(B/B o). If B is attenuated by 1 db (which corresponds to a 100 mvdc delta on the voltmeter), then: ( B (@ 1dB) / B ) (A - 6) 1dB = 20Log o B ( ) ( ) (@ 1dB,100mV ) B B 10 1/ = = B / p p So, time domain hum modulation can be defined as: O O O ( ) V (A - 7) p p Env Hum(dB) = 20Log max Env min Env max (A - 8) Where Env max is the 1 db delta in B (envelope change) that can be compared to the actual peak-to-peak hum voltage (Env max-env min) to give a hum modulation ratio in db. [ B (@ 1dB, 0.1Vp p )] [ ] (A - 9) Env max = ( 1/ ) Env max Env min = Hum p p (A -10) At the 100mVDC calibration reference (which is a 100 mv p-p delta envelope reference), the equation for hum modulation is: ( 1/ 20) Hum Hum(dB) = 20Log(1 10 ) + 20Log mvp p 100mV p p ref (A -11) Where Hum p-p is given in millivolts, and hum modulation is given in db. AMERICAN NATIONAL STANDARD SCTE 16

17 Hum(dB) = Log 100mV ( ref ) + 20Log(Hum ) = Log( Hum ) (A -12) p p mvp p + mvp p AMERICAN NATIONAL STANDARD SCTE 17

18 Appendix 3 Conversion of Hum Modulation from db to percent (%) The Federal Communications Commission (FCC) defines hum in Technical Standards part (a) as: The peak to peak variation in visual signal level caused by undesired low frequency disturbances (hum or repetitive transients) generated within the system, or by inadequate low frequency response, shall not exceed 3 percent of the visual signal level. This appendix will detail the derivation of hum modulation in percent, and will provide a table with conversions of hum modulation from mv p-p to db and percent hum. Hum Modulation may be expressed as a percentage of the peak to peak variation of the carrier level to the peak voltage amplitude of the carrier. From equations A-9 and A-10, located in Appendix 2, Env max is defined as the 1 db delta in B (envelope change) that can be compared to the actual peak to peak hum voltage (Env max - Env min, or Hum p-p) to give a hum modulation ratio. To express hum modulation as a percent: Hum(%) where, Env max HummVp p HummVp p = *100% *100% =.109 * HummVp p Env = (A-13) max 919.5mV p p 100mV 100mV = (from A-9) p p p p ( 20) p p [ ] 1/ = = 919.5mV The 100mV delta is shown as the standard reference. However, for hum measurements, the actual delta of voltages between the 1 db change should be used for improved accuracy. The conversion of hum in mv p-p to db and percent is shown in Table 1. AMERICAN NATIONAL STANDARD SCTE 18

19 Table 1 - Hum Modulation Conversion Chart mv p-p db % mv p-p db % mv p-p db % Hum (db) = log (Hum mvp-p) Hum (%) = * Hum mvp-p AMERICAN NATIONAL STANDARD SCTE 19

20 Appendix 4 Time Domain vs. Frequency Domain Measurements This appendix is intended to demonstrate a mathematical correlation between the time domain measurement method (which this procedure details), and the frequency domain measurement method. The frequency domain method uses a spectrum analyzer as a tuned receiver to demodulate the 60, 120, or 180 Hz hum sidebands to baseband for measurement with a low frequency spectrum analyzer. This measurement technique is seldom used as it requires a spectrum analyzer with 10Hz or better resolution. Before a correlation between the two measurement methods is shown, the equations for conventional AM and TV AM should be reviewed. Conventional AM: x(t) = A 1 Where A is the carrier and m is the conventional modulation index. The equation for TV AM (A-1) is located in Appendix 1. If these equations are converted to show the AM sidebands: Conventional AM: x(t) TV AM: y(t) = B 1 [ + m( cosω m t) ] cosω t (A -14) = A cosω c t c [ ( ω ω ) t + cos( ω + ω ) t] (A -15) m cos c k 4 m c It can be determined from these equations that for conventional AM signals, the AM sidebands are m/2 volts(assuming that carrier A is 1 volt). For TV AM, the sidebands are k/4 volts(also assuming that carrier B is 1 volt). Appendix 1, through mathematical derivation, shows that Hum(dB) = 20log(k). If the frequency domain method is used to measure the sideband, the equation becomes: Hum(dB) Therefore, for frequency domain measurements, a correction factor of 12 db must be subtracted from the measurement to correlate to the time domain measurement method. m [ ( k / 2) ] cosω t [ cos( ω ω ) t + cos( ω + ω ) t] (A -16) = 20Log c c m c ( k ) = 20Log(k) 20Log4 = 20 log(k) 12 (A -17) 4 m AMERICAN NATIONAL STANDARD SCTE 20

21 Appendix 5 - Hum Modulation Test Methods Note that both of the time-domain methods detailed in the procedure depend upon the comparison of an unknown envelope to a known calibrated envelope. The difference between the two methods lies in the way the reference envelope is established and measured. The general principles of hum modulation measurement are covered in Appendix 1. 1 db Delta Method The 1 db delta method creates a standard reference envelope by switching a 1 db attenuator in the signal line to the device under test, and calibrates the amplitude of this 1 db envelope by measuring the rectified DC component of the carrier with a digital voltmeter. The oscilloscope is not involved in this calibration. This procedure presumes that a large signal DC measurement correlates to a small signal AC measurement using a different instrument, and it presumes that the AC/DC characteristics of the detector are constant for all test frequencies. The derivation of the formula for hum modulation using this method is found in Appendix 2. The accuracy of this method is influenced by the accuracy of the 1 db attenuation, the frequency and signal level response of the AM detector, the gain of the differential amplifier, and the correlation of the measurement instruments measuring different signal components. This kind of instrument diversity makes this an absolute measurement. Differential Voltage Method The differential voltage method creates a standard reference envelope by deploying the internal AM modulation function of the generator as described in 5.6 to establish calibration verification. This reference modulation envelope is measured as a peak to peak AC voltage waveform on the oscilloscope in the same way that the hum modulation waveform is measured using the same measurement instrument. The rectified DC component of the signal does not enter into the measurement, therefore, the detector response with respect to frequency does not affect the measurement. Likewise, the gain and insertion loss accuracy in the signal path does not influence the result because it is common to both the reference and the test measurement. The accuracy of this method is influenced by the calibration of the modulation index of the internal modulation generator, and the vertical range selector of the oscilloscope. This kind of instrument commonality makes this a releative measurement. In establishing a relation between the hum waveform and the peak carrier level, an intermediate reference waveform is used which has a known relationship to the carrier by virtue of its modulation index. From Appendix 1, the relationship of a conventional AM modulation envelope defined by its modulation index m, to the peak carrier amplitude B is given by: AM mod(db) = 20Log ( kb 2m ) = 20Log( k), where k = (A - 3) The relationship of the peak to peak hum waveform to the peak to peak reference modulation is simply: V hum 20 Log as measured on the oscilloscope. Vref (A 18) B 1+ m AMERICAN NATIONAL STANDARD SCTE 21

22 Combining these two relationships, the peak to peak hum is related to the peak carrier by: V hum Hum (db) = 20Log(k) + 20Log or (A 19) Vref Hum(dB) = 20Log(k) 20Log(V ) 20Log(V ). (A 20) ref + A convenient modulation index to use is 0.5% AM. If this value is substituted for m, the first term in equation A-20 becomes db. If a higher degree of accuracy is required, the modulation index can be empirically determined by measuring the carrier and its sidebands across the test spectrum. From HP application note #150-1, the relationship between the logarithmic display and the modulation percentage is expressed as: (db) C(dB) hum 20log m = E SB E + 6dB (A - 21) m = 10 ( ESB EC + 6) / 20 (A 22) AMERICAN NATIONAL STANDARD SCTE 22

23 Appendix 6 - Troubleshooting Guide This appendix will serve as a guide to eliminate AC ground loops in the hum modulation test setup. Identification of a ground loop problem will normally be made when measuring the back-to-back hum modulation of the power combiners. If the noise floor is not lower than 75 db, the setup should be investigated for possible contribution to the noise floor. General Troubleshooting If the noise floor goal cannot be met, take the following steps to determine if the test equipment/test setup is contributing to the problem: 1. Ensure all test equipment is set to the operating parameters detailed in this procedure. 2. Ensure all connectors and cables used in the test setup are correctly constructed and firmly connected. Light shaking or tapping on each connector can help to identify a potential problem. 3. Ensure the hum power combiners are constructed properly; i.e. check for poor solder connections, etc Ground Loop Elimination If the conditions in the general troubleshooting guide are met, an AC ground loop is most likely the cause of the poor noise floor measurements. The following steps may be taken to prevent or eliminate these ground loops. 1. Utilize a power bar (driven from an isolation transformer) with the common ground removed on each piece of test equipment (signal generator, oscilloscope, and voltmeter). 2. Isolation plugs (2 3 prong adapter type) should be used on each piece of test equipment. 3. Ensure that any plug outlet being used for the test is not introducing any 60 Hz hum to the system. 4. Avoid multiple ground paths between equipment such as the signal generator and the oscilloscope. A cable connecting the generator modulation test point to the oscilloscope external trigger must not have a complete ground path 5. Special Consideration: If the test setup is automated, ground loops can occur between the PC and any test equipment via the instrument interface bus. Bus opto-isolators can be utilized to eliminate this problem. AMERICAN NATIONAL STANDARD SCTE 23

24 Appendix 7 Special Test Conditions/Considerations The test setup and procedure in this document detail a test methodology that is specific to devices in which the power enters and exits on the RF test ports. However, some test devices do not contain this configuration. Drop amplifiers are an example of a device that does not contain two RF/power ports. Typical configurations of drop amplifiers include use of only one RF/power port, or a dedicated power port and stand alone (no power passing capability) RF ports. Under circumstances such as the ones listed above, this procedure can be followed with minor test set-up modifications. If one or more RF ports of the DUT do not contain power passing capability, AC/RF power inserters/combiners are not required for those ports. They may be removed for testing purposes. For example, a DUT containing only one RF/power port would be tested using one AC/RF power inserter/combiner on the RF/power port, and an RF only connection on the other port. A DUT containing a dedicated powering port would not require AC/RF power inserters/combiners on the test ports, as they are not meant to pass power. A general rule of thumb when performing this test is to utilize AC/RF power inserters/combiners on any DUT port that shares RF and power passing capability. Otherwise, direct connections should be used. Additionally, the power load bank is not required on DUT s without power passing capability. The procedure can be followed as stated with these minor changes to the test set-up. AMERICAN NATIONAL STANDARD SCTE 24